Differential mobility analyzers are known based on establishing a drag flow with high Reynolds numbers and the smallest possible degree of turbulence through which a target particle is made to cross.
This particle is injected in a perpendicular direction with an electric charge obtained after an ionization stage.
The presence of an electric field perpendicular to the flow direction drives the particle through the cross flow to a greater or lesser degree given the value of electric mobility which depends on the charge and diameter of the particle among other parameters.
Given that the particle is dragged downstream by the main drag flow, the greater or smaller velocity of the particle according to its electric mobility will give rise to the point on which it strikes on the other side of where it has been injected being located at a greater or smaller distance.
The impact at a greater or smaller distance may be read by means of a multisensor which detects the exact location of this impact in the longitudinal coordinate, the one that follows the flow. The electric mobility of the particle is a function of the distance where the impact occurs.
Another alternative is that of incorporating an exit slot. If this slot is located at the distance at which the impact of the target particle occurs, that which is intended to be detected, the target particle entering the mobility analyzer will cross it according to the trajectory reaching said slot such that the particle may be extracted.
Thus, not only its presence is detected but it can be taken via devices of greater accuracy which reduce the threshold of uncertainty on the value of its electric mobility.
This is the way in which the increase of resolution has been carried out in the state of the art, the incorporation of devices at the exit of the analyzer; in particular, PCT patent application with number 2005/ES070121 is mentioned.
Publications such as [“Drift differential mobility analyzer”, J. Aerosol Sci., Vol. 29, No. 9., pp. 1117-1139, Ignacio G. Loscertales], wherein the influence on the resolution of the DMA of the presence of an oblique electric field (E), such that, apart from the transverse component Ey of the field, there is a non-zero component Ex (with regards to the main drag flow) and in the direction opposite to said flow, are known.
This study is a theoretical analysis where the increase of the resolution of the DMA is linked according to the oblique electric field E, in particular of its non-zero component Ex.
The mathematical development of this analysis utilizes a dimensional variables X, η. These a dimensional variables are defined as =x/b, η=y/b, where x is the non-zero component that follows the drag flow, y is the coordinate transverse to the flow, and b is the separation distance between the two walls between which the trajectory of the particle is established. By denoting the electric field (E) according to the a dimensional variables, now its components are expressed as E=(f, fη).
The results of this analysis determine that the error reduction factor is of the order of
      (                  E        x                    E        y              )        (          1      /      2        )  
In particular, when the electric field is expressed according to the coordinates X, η, then the reduction factor may be evaluated from the value
  K  =            ∫      0      1        ⁢                  (                                            E              x                        ⁡                          (              η              )                                                          E              y                        ⁡                          (              η              )                                      )            ⁢              ⅆ        η            such that the increase factor on the resolution of a DMA utilizing the oblique electric field with regards to another that does not may be expressed as 1/√{square root over (2K)}.
This expression means that the increase of the value of K reduces the error reduction factor; and also, that the resolution may be, at least theoretically, increased without an upper elevation as much as desired. This decrease is proportional to the non-zero component Ex, and the greater the inclination angle of the electric field (E) the larger the latter will be.
The detailed study of this factor K and of the equations leading to its deduction also allows to ensure that the resolution increase is only obtained if Ex is counter-currently oriented.
This study is focused on the mathematical analysis that leads to said conclusions and does not explain how this oblique electric field may be obtained in practice. However, an attempt to obtain a device with a narrow oblique electric field (E) region which utilizes a pair of grids parallel to one another, arranged oblique in the midst of a drag flow, the work area being limited to the places between the grids in which the oblique electric field (E) is ensured, giving rise to very bulky devices in which the effective volume is very reduced, is known. Another serious drawback it has is the interference of the wake of the grids on the drag flow.
The present invention defines a device utilizing properly selected and configured electrodes such that the whole of the analysis region, except for edge effects, has an oblique field (E) without distortions of the latter or of the drag flow as it does not include elements immersed in the midst of the flow.